According to current standards applied in many countries with regard to breathing apparatus intended for use in contaminated environments and in fire environments, such apparatus is required to be provided with a suitable warning device which will warn the wearer of the apparatus that the pressure in the gas reservoir has reached a predetermined value.
For instance, in current European standards for bodycarried breathing apparatus operating with compressed air, the aforesaid predetermined pressure level is given as 50 to 60 bars or when at least 200 liters free inhalation gas remains in the gas bottle or gas reservoir. Other pressure values apply in U.S. standards (U.S.A.) for instance. The majority of commercially available breathing apparatus of the kind intended here are provided with an acoustic warning device which, at the applicable predetermined warning pressure, activates a whistle which is operated by the gas from the gas reservoir.
Different standards specify the frequency and sound level of the acoustic warning signal produced.
Since it is possible that the person wearing the breathing apparatus is located in a high sound level environment and may, in certain cases, wear a head guard, such as a helmet and/or a protective hood which restricts the ability of the wearer to hear the acoustic warning signal, or the hearing of the wearer may be impaired, it is possible that the wearer will fail to hear the acoustic warning. When the person concerned is working in a contaminated environment or a fire environment, his/her failure to hear the acoustic warning may prevent the person from stopping work in time to ensure a safe return. Conversely, the aforesaid person may misinterpret some other sound, such as an acoustic fire warning signal, and prematurely interrupt a life-saving operation in the belief that the acoustic warning of his/her breathing apparatus has been activated.
The present invention can also be applied to pneumatically operated systems in general, where a predetermined pressure is indicated by an acoustic signal.